The tables below give dimensions for threaded bottom-bracket shells and parts which thread into them. These parts can be:

internal or external bearing cups,

Phil Wood and other mounting rings for cartridge bearings;

threaded cartridge-bearing assemblies.

As of 2015, cartridge bearings are being used for most bottom-bracket bearing assembly replacements. Cartridge-bearing installation is simple; assemblies are available in several brands at various price points and with various spindle lengths. If you have installed a cartridge bottom-bracket bearing assembly with a wrong length axle, substituting another is easy and you aren't stuck with greasy surplus parts. Also see our article about cartridge-bearing bottom brackets.

You may still want to use a traditional cup-and-cone bottom bracket for authentic restoration of an older bicycle, or because you happen to have parts on hand. Bearing dimensions of spindles for traditional cup-and-cone bottom brackets varied enough among brands and models that the best advice is to use complete bottom-bracket sets made to work: see our bottom-bracket size database for comparisons among these. Mixing and matching cups and spindles can have an unexpected effect on chainline, or may not even be possible. In case you need dimensions of unusual bottom-bracket parts, or need to mix and match, the 6th edition of Sutherland's Handbook for Bicycle Mechanics deals with these issues in great detail. Any good bicycle shop will have a copy

British and Italian-threaded bottom-bracket cups are easily available; French ones less so, though still in production as of 2015. Raleigh cups have to be scavenged -- though they are common. Common J.I.S square-taper bottom-bracket spindles can be used with French or Raleigh cups, though with Raleigh cups, it may be necessary to use 6 mm or 15/64" bearing balls. See our article about Raleigh dimensioning issues and our one about French bicycles. Phil Wood cartridge-bearing mounting cups are made for every type of bottom-bracket threading. This is the expensive but headache-free way to go. Velo-Orange makes a French-threaded cartridge-bearing assembly, and a threadless one which clamps into the bottom-bracket shell. This will work with any bottom-bracket shell of the usual 68 mm length, even if the threads have been stripped..

Shimano spec, measured to the midpoint between the rings.
with typical 5 mm chainring spacing, this puts the inner at 41 mm, the outer at 46 mm.

Road Triple

45

Shimano spec, measured to the middle ring.

MTB Triple

47.5-50 mm

Shimano spec, measured to the middle ring.
47.5 preferred, but for frames with oversized seat tubes, the longer dimension may be needed, because the fat tube places the derailer mechanism farther to the right.

Track/Coaster Brake
Traditional One-Speed
Most internal gear hubs

40.5-42 mm

Older bikes with 110 spacing would be on the smaller end of this range
Newer bikes with 120 mm spacing normally use 42 mm

Singlespeed MTB

52 mm

Wider chainline need for chainstay clearance on MTBs.

This is close to the chainline of the outer ring of a typical MTB triple

Singlespeed MTB
Alternate

47.5 mm

White Industries ENO hubs use this chainline, which lines up with the middle position of a typical MTB triple.
It's also fairly close to the outer position of a typical "road" double.